Traffic Noise Model: Frequently Asked Questions FAQs

Menu: Input

Barriers

Why does TNM give a flat top berm warning when I enter in a non-zero berm top width value?

(Applies to TNM version 2.5 only)

A new pop-up warning box is generated each time a user enters a non-zero top-width for berms, which warns against using flattop berms in TNM. This prohibits the user from modeling a flattop berm with the barrier/berm function. For information on why it is recommended that flat top berms not be modeled, please read the FAQ below "Should I model my berm as a wedge or with a flat top?"

Should I model my berm as a wedge or with a flat top?

In all versions of TNM, it is recommended that TNM users only use wedge berms (a default berm top-width of 0.0), because TNM had shown some apparent anomalies in the diffraction algorithms for berms with a top width (flat top berms). These anomalies may also surface for other TNM geometries that produce terrain shapes similar to flat top berms (i.e. rising slope, leveling off, and then recessing slope). This problem does not manifest itself for geometries that come to a peak, and in many instances, flat top berm geometries can be effectively modeled with wedge berms.

If modeling flat-top-berm-like geometries with terrain lines, it has been found that by rounding-off any sharp corner geometries in TNM with 2 terrain lines per corner instead of just 1, the anomalies in the diffraction algorithms associated with flattop-berm-like geometries can be minimized and even eliminated. This has been shown to improve results as long as the slope of the "rounded" corner is not too shallow (similar to the flat top; 5:1 run to rise, or greater) or steep (steeper than the slope of the berm).

What are the three ways of entering barriers that will affect the sound level results?

Perturbable-height barrier: Only affects the sound level in the "with barrier" case. Barrier attenuation is not included in the "no barrier" sound level. The "no barrier" and "with barrier" sound levels will be different. Perturbable-height barriers allow for multiple adjustments to the barrier height (one of them being to a height of zero) when performing a barrier analysis. To implement: set the "#up," "#down," and increment size to non-zero values.

Single-height barrier: Only affects the sound level in the "with barrier" case. Barrier attenuation is not included in the "no barrier" sound level. The "no barrier" and "with barrier" sound levels will be different. Single-height barriers allow for only one adjustment (to a barrier height of zero) when performing a barrier analysis. To implement: leave the "#up" and "#down" perturbations at zero, but make the increment size non-zero.

Fixed-height barrier: affects the sound level in both the "no barrier" and "with barrier" cases. Barrier attenuation is included in the "no barrier" sound level, which will equal the "with barrier" sound level if there are no other perturbable or single-height barriers in the run. These barriers are generally used for permanent structures, such as an existing wall, bridge parapet, or a crash barrier; they may not be perturbed in barrier analyses. To implement: set all three perturbation values to zero.

How and when do I model Jersey barriers?

If you model Jersey barriers, use fixed-height barriers; this applies to roadside or median jersey barriers. Be careful when modeling median jersey barriers; TNM does not currently account for reflections back across the roadway (except in the parallel barrier module).

How does TNM calculate insertion loss?

When a barrier is present in a TNM run, TNM accounts for both the sound reduction generated by the diffraction over the barrier and ground effects for any sound that reaches the ground between the barrier and receiver (where TNM accounts for the shadow zone). The net effect of barrier diffraction, combined with the partial loss of the ground effect, is the barrier insertion loss.

TNM does not currently incorporate the existing background level in determining the final insertion loss; TNM users must determine when it is appropriate to incorporate the background levels. The background level can be measured in the same or similar neighborhood at a distance from the roadway where the highway traffic noise is not heard. TNM's predicted "with barrier" level can be logarithmically combined with the background level (in a simple process in a separate spreadsheet) to determine the insertion loss. For information on how to add background noise please see the FAQ, "How do you add background noise to TNM results?" under the Calculate menu.

At far distances, the existing background noise is typically at higher sound levels than the received highway noise, so accounting for background noise is essential. There are cases where meteorological effects (e.g., wind speed and direction, temperature lapse or inversion) can play a major role in the received sound levels, causing the highway noise to exceed the background noise. Since TNM does not currently account for these meteorological effects, users must apply good engineering judgment when modeling receivers at far distances.

How does ground type affect insertion loss?

Here are some examples of how ground type can affect insertion loss:

When removing absorptive ground cover between a barrier and a receiver, you are removing the loss of soft ground attenuation in the shadow zone and should therefore increase the insertion loss.

When replacing ground cover with an acoustically more absorbent ground cover between a barrier and a receiver, you are increasing the loss of soft ground attenuation in the shadow zone and should therefore decrease the insertion loss.

I'm perturbing the barrier up and down in the barrier view, but why aren't the results in the tables changing?

Barrier View must be the active window when you open all other barrier-related windows, to link windows together. Otherwise, tabulated results will not change as you shift barrier heights up/down. Check the Barrier Design name in the table; if it reads "Input Heights", close the table, click on Barrier View and then re-open the table.

I started to do a simple test run, which passes the input check and then immediately gives a "Segmentation Violation" dialog and stops the calculation. What does this mean?

There are certain TNM object placements that will cause TNM to overflow. Because TNM performs many of its geometric calculations by drawing triangles along the source-receiver path, division by zero calculations are possible if the extensions of the objects are pointing at receiver locations.

Overflows also are more likely to occur if there is a high density of path-oriented objects (e.g., terrain lines, building rows), such as two terrain lines too close together. When deciding how close your terrain lines should be, always take line-of-sight into account. For example, cases involving noise barriers may have different elevations in front of and behind the barrier; if not modeled properly, the highest vehicle source (at a height of 3.66 m [12 ft.]) could be inadvertently blocked by the barrier or exposed to the receiver. In general, use good engineering judgment to simplify your geometry.

If TNM outputs a segmentation violation message, don't click OK right away; make a note of which receiver caused it to crash, and try moving the location slightly, or disabling it in future calculations.

No, TNM does not. While this is a very feasible occurrence in real life, TNM does not allow barriers to go underneath structure-roadways (or barriers), and will produce "invalid" receivers every time a barrier goes underneath a roadway/barrier on-struct (instead of stopping the calculations). To avoid this error, stop your barrier just shy of the structure roadway, and then start it up again on the other side.

If you enter a value of 0 for all of the perturbation parameters (Increment, #Up, and #Down), TNM will designate the particular barrier as a fixed-height barrier. To designate a design barrier without perturbations (single height barrier), enter values in the Increment column. To designate a design barrier with perturbations (perturbable-height barrier), also enter values in the #up and/or #down columns.

Does TNM account for sound being reflected off a barrier back over the roadway?

No. This feature is not yet implemented in TNM. As a result, the columns, "Reflections ?", and the associated next three columns are grayed out and not available for selection/editing. To see how some users have modeled sound being reflected off a single barrier, please refer to the FAQ in the Parallel Barrier section regarding single wall reflections.

Why doesn't TNM 1.1 recognize my berm design?

(Applies to TNM version 1.1 only)
TNM 1.1 ignores berms in calculating sound level results. Please use TNM Version 1.0b or 2.0 (or later versions) when calculating runs with berms.

TNM outputs "invalid" results listed in the Sound Levels results table when I model berms. What is causing this error message?

The toe of the berm (end of slope) can bury receivers and it may not be obvious. This will cause "invalid" results for that receiver. Use the skew view to make sure the toe of the berm is not overlapping a roadway or receiver location.

How does TNM calculate Barrier Noise Reduction?

TNM calculates barrier NR as the difference in sound level before and after installation of a barrier, where the source, ground, and atmospheric conditions have been judged as equivalent. NR accounts for both the barrier shielding effect and the loss of ground effects in the shadow zone behind the barrier. For sound that reaches the ground behind a barrier, TNM accounts for ground effects. Note that TNM's Barrier Noise Reduction (NR) is the term used for Insertion Loss (IL).

Keep in mind that TNM does not currently include background or existing sound levels in its NR calculations. You should always consider background sound levels when presenting final NR values, especially when receivers are placed a far distance from the roadway(s). Background sound levels should be logarithmically added (see equation below) to your predicted sound levels in order to obtain accurate NR values.

Also, keep in mind that TNM does not currently include meteorological effects such as wind and temperature gradients in its NR calculations. These meteorological effects can influence sound levels at far distances.

Equation to add two dB levels (C = A + B) ,where A, B, and C are all expressed in decibels:

C = 10log[10(A/10) + 10(B/10)]

How do I model concrete retaining walls?

Model a fixed height barrier representing your retaining wall (no perturbations), then put a terrain line a few meters (1.52 m - 3.05 m [5-10 ft.] should work fine) behind it at an appropriate elevation. Also, if you view a cut section, you can see the terrain line starts at the barrier's bottom elevation(s), and sharply go up to the location of the elevation input for the terrain line. This method should eliminate the "false" ground attenuation obtained with the dual terrain line method. An example is illustrated below:

Should I model berms that are median barriers?

Yes. If a berm is a median barrier then it should definitely be modeled. Median barriers should be modeled as fixed-height barriers, unless the barrier is part of a noise barrier design, which is uncommon.

A fixed-height barrier is a barrier that affects the sound level in both the "no barrier" and "with barrier" cases. Barrier attenuation is included in the "no barrier" sound level, which will equal the "with barrier" sound level. Fixed-height barriers are generally used for permanent structures, such as an existing wall; they are ineffective in barrier analyses. To implement: set all three perturbation values to zero.

A Berm Top Width currently needs to be equal to zero. Do not use flat top berms.

In the Shielded List for structure barriers, why does it appear to have text and fields pertaining to adjustment factors?

(Applies to TNM version 2.1 and 2.5)
This is a bug in TNM 2.1 and 2.5 that displays text regarding adjustment factors in the Shielded List. The data in the fields are correct. Ignore the text regarding adjustment factors and add roadways/segments to the shielded list as you normally would.

Which roadway segments can be shielded by structure barrier segments?

Structure barrier segments may shield both structure roadway segments and non-structure roadway segments. They are assigned by the user in the Barriers input dialog box under Structure, Edit*. When in doubt, assign more roadway segments rather than fewer as being potentially shielded by each structure barrier segment. At a minimum, include adjacent roadways that are shielded. Also, it is good practice to not assign the shielded roadway segments to the structure barrier segments until all graphical editing has been completed. Scrambling sometimes occurred in TNM 1.0b, that is, it may assign different (incorrect) roadway segments to the structure-barrier segments. If you graphically add more roadway segments to the shielded list after initial assignments, you must reselect the previously assigned ones, or they will be removed from the shielded list for that barrier segment.

*Note that there is a bug in TNM 2.1 and 2.5 that displays text regarding adjustment factors in the Shielded List. The data in the fields are correct. Ignore the text regarding adjustment factors and add roadways/segments to the shielded list as you normally would.

What does an "HPP Count Mismatch" error message mean?

This error can be caused when there is a barrier underneath a structure roadway. Remove the barrier or split the roadway into 2 roadways with a small gap between them to let the barrier "pass through" in the plan view and re-calculate the run. Note that a barrier's benefit might be nullified by underdeck reflections, which are not accounted for in TNM.

What does a "segment skipped" error message mean?

This error can be caused by a barrier that overlaps a roadway surface. Use View, New View, Skew Section to search around the obviously possible places, then move the overlapping barrier further from the roadway centerline.

Why am I able to shift a fixed-height barrier to zero height?

There is a bug in barrier view that incorrectly allows you to shift fixed-height barriers to zero height. Doing so will incorrectly reduce the cost to zero but acoustical calculations will be correct (input height).

How long should my barriers be?

A rule-of-thumb is that a barrier should be minimally 8 times the distance between the barrier and the most-distant receiver, with the receiver centered along the barrier.

How do I know which is the left or right side of a barrier?

Each barrier has a left and right side, depending upon the direction you input it. As you walk along the barrier in the direction of input, the left side of the barrier is to your left, the right side to your right. In all graphical views, TNM draws barriers as arrows, to show their input direction and thereby their left and right sides. You may have to zoom in on a barrier to see the arrows.

Does TNM calculate multiple perturbable barriers simultaneously?

TNM does not calculate more than 2 perturbable barriers simultaneously between a roadway and the closest receiver. TNM will only choose the two most effective perturbable barriers based on their INPUT heights (not the perturbable heights) and calculate levels for those barriers only. Please refer to the Perturbable barrier reduction section on Page 77 of the TNM tech manual.

What steps should I take to place a wall on top of a berm?

When placing a wall on top of a berm, you must have the same number of segments, going in the same direction with the exact x-y coordinates, or TNM will output an error message and stop sound level calculations. The Z-bottom of the wall barrier must equal to the Z-bottom of the berm plus the berm Height at each point. When a wall is placed on the berm, the berm cannot be perturbed. Also, when putting walls on top of berms, type in coordinates (do not use snap tool).

To include the berm of the berm/wall combination in the "no barrier" case (i.e., the berm is not part of the abatement feature), set all of its perturbation parameters (increments, up and down) to zero, making it a fixed-height barrier.

If the berm of the berm/wall combination is going to be part of the abatement, leave the up and down perturbations at zero (making it a single-height barrier), but include a non-zero number in the increment box; the berm's noise reduction will not be included in the "no barrier" results. However, during barrier design, you may only perturb the wall barrier on top of the berm. The berm should remain at a constant height throughout design. To change the berm height, edit the berm input height and the Z-bottom of the corresponding wall barrier, and re-run. Previous "remembered" Barrier Designs will be invalid.

What are the limitations for using TNM to design noise barriers?

A basic assumption of the FHWA TNM is that highway noise dominates the noise environment. Adding other sources, such as other roads in the surrounding roadway network, helps fill in more information about background noise levels, but other sources may exist in the environment that TNM does not include. Some users use post-processing methods to add in background noise during the barrier design process to verify the effectiveness of barrier designs from TNM, while others use the results from the barrier design tool in the model. Both approaches are valid. Based on the results of the TNM validation study and other related research, it is reasonable to limit noise barrier design to receptors within 500' of the edge of the highway.

Building Rows

How precise should I be when considering the height of building rows?

Generally, plus or minus 2 m (6 ft) is precise enough for average height, as long as most buildings are within a story of one another and as long as the road is not on fill where the height may become more critical.

How about precision in determining building spacing?

It is not necessary that building spacing be highly regular. You do not have to be overly precise in this parameter: +/- 10-20% should be good enough.

Does TNM account for the actual locations of gaps along a building row?

TNM does not account for the actual location of gaps along a building row. For this reason, it predicts average results behind the building row -- that is, average of locations directly behind gaps and locations directly behind building structures.. If you wish more precise results, you must input each individual building structure as a separate TNM barrier. For example, consider modeling buildings as separate TNM barriers for model calibration runs for measuring sites beyond the first row of houses.

Should I input a building row or a barrier?

TNM does not allow you to enter a building percentage greater than 80 percent. If your building percentage is greater than this, input a noise barrier instead of a building row. Please refer to the previous Q/A, "Does TNM account for actual locations of gaps along a building row?", for more information.

How much attenuation does TNM contribute for building rows?

In the case of multiple building rows, TNM first identifies all building rows that interrupt the effective source-receiver path. Rows that do not interrupt the propagation path are ignored. For each row that interrupts the path, TNM determines which building row has the most effective attenuation at the 630 Hz frequency band. For this building row, the actual attenuation is calculated for all 1/3-octave frequency bands. For each remaining row that interrupts the propagation path, an attenuation of 1.5 dB is applied to each 1/3-octave band. The maximum attenuation for any number of building rows has been set to 10 dB. For a listing of maximum attenuation for each 1/3-octave band please refer to Table 13 on Page 100 in the TNM Tech Manual.

The Building Row dialog is cutoff on the right so I can't enter a building row percentage.

You will need to set your computer monitor's resolution to 1024 x 768 or greater, 16 colors or greater, and the smallest available font size. You can usually accomplish this in the computer's Control Panel, Display, Settings function.

Terrain Lines

Should I use terrain lines as often as possible?

No. Minimize the use of terrain lines. Don't include terrain lines for small ground inflection points. In general, model terrain lines when there are ground undulations of 1.52 m (5 ft.) or greater. If a more detailed terrain analysis is desired, smaller ground undulations can be included. Always take line-of-sight into account when choosing to model a terrain line. As an example, cases involving noise barriers may have different elevations in front of and behind the barrier; if not modeled properly, the highest vehicle source (at a height of 3.66 m [12 ft.]) could be inadvertently blocked by the barrier or exposed to the receiver. In general, use good engineering judgment to simplify your geometry.

Terrain Lines should also be used to define the ground:

At the toe of a slope, for roadways on extensive fill.

At the top of a slope, for depressed roadways.

Just off the structure of roadways on structure.

At the top edge of retaining walls, for roadways in cut.

How does TNM handle terrain lines?

Insert terrain lines wherever you wish TNM to know the general height of the terrain between source and receiver. However, please note that small changes in terrain elevations have little effect on the final sound levels. TNM's ground-smoothing algorithm will automatically "smooth" away minor elevation changes prior to sound level calculations to reduce computation time. TNM has shown some weaknesses in the diffraction algorithms, which would be invoked when computing the effects of terrain lines (This has been corrected in TNM version 2.5). Please refer to the previous terrain line FAQ, "Should I use terrain lines as often as possible?" for guidance on the use of terrain lines.

Where should I put terrain lines?

Typical terrain lines might lie: Along the bottom of deep (1.52 m (5 ft.) or greater) drainage ditches that parallel the roadway; Along the top edge of slope for depressed roadways; Along the edge of fill for roadways on fill or on structure; Along the ridge of an intervening hill; Along a line of constant elevation (top of line) of an intervening hill. Please refer to the terrain line FAQ above, "Should I use terrain lines as often as possible?" for further guidance.

Where should I not put terrain lines?

It is not necessary to enter terrain lines where the Z coordinates of other input serve to define the topography, such as: Z (pavement) of roadways, at the edge of pavement rather than at the roadway centerline; Z (ground) of receivers; Z (bottom) of barriers, including berms; Z (bottom) + Height of berms, but at the edge of the berm top width, rather than at the berm centerline; Z (ground) of building rows; Z (ground) of the perimeter of tree zones.

Should I model my berm as a wedge or with a flat top?

In all versions of TNM, it is recommended that TNM users only use wedge berms (a default berm top-width of 0.0), because TNM had shown some apparent anomalies in the diffraction algorithms for berms with a top width (flat top berms). These anomalies may also surface for other TNM geometries that produce terrain shapes similar to flat top berms (i.e. rising slope, leveling off, and then recessing slope). This problem does not manifest itself for geometries that come to a peak, and in many instances, flat top berm geometries can be effectively modeled with wedge berms.

If modeling flat-top-berm-like geometries with terrain lines, it has been found that by rounding-off any sharp corner geometries in TNM with 2 terrain lines per corner instead of just 1, the anomalies in the diffraction algorithms associated with flattop-berm-like geometries can be minimized and even eliminated. This has been shown to improve results as long as the slope of the "rounded" corner is not too shallow (similar to the flat top; 5:1 run to rise, or greater) or steep (steeper than the slope of the berm).

No, TNM does not. While this is a very feasible occurrence in real life, TNM does not allow terrain lines (or barriers) to go underneath structure-roadways (or barriers), and will produce "invalid" receivers every time a terrain line goes underneath a roadway/barrier on-struct (instead of stopping the calculations). To avoid this error, stop your terrain line just shy of the structure roadway, and then start it up again on the other side.

Ground Zones

When should I model grass medians?

(Applies to TNM version 1.0, 1.0b, 1.1, 2.0, and 2.1)
Users should model grass medians when the median is greater or equal to 3.05 meters (10 ft.). If you are using a default ground type of field grass, create a ground zone of field grass between two roadways to model a grass median. If you are using any default ground type other than field grass, create a ground zone of lawn between two roadways to model a grass median.

For all roadways, it is necessary for the median to overlap the roadway edges as defined by their widths because TNM will default to the pavement until it reaches the edges of the roadways. For medians less than 10 ft, use 1 roadway without traffic to model the median or use 2 wide roadways that overlap. Also, do not snap ground zone points to the roadway points when modeling the median. Often check that they are properly overlapping using the Skew Section view.

How do I model roadway medians?

(Applies to TNM version 2.5 only)
The recommendation on how to model medians has changed from previous versions of TNM. For TNM Version 2.5, the following recommendations apply:

the minimum recommended width is still 3.05 m (10 ft);

if the median is a ground type other than the default, use a ground zone to model the median; and

for a ground zone median, avoid overlapping or matching edges with the adjacent roadways.

How do I model roadway medians?

(Applies to TNM version 1.0, 1.0b, 1.1, 2.0, and 2.1)
Users should model roadway medians when the median is greater or equal to 3.05 meters (10 ft.). If you are using a default ground type of field grass, create a ground zone of field grass between two roadways to model a grass median. If you are using any default ground type other than field grass, create a ground zone of lawn between two roadways to model a grass median.

For all roadways, it is necessary for the median to overlap the roadway edges as defined by their widths because TNM will default to the pavement until it reaches the edges of the roadways. For medians less than 10 ft, use 1 roadway without traffic to model the median or use 2 wide roadways that overlap. Also, do not snap ground zone points to the roadway points when modeling the median. Often check that they are properly overlapping using the Skew Section view.

Does a tree zone automatically define the ground type?

No. If you want a ground type other than the default, you must use a ground zone to define the type of ground inside tree zones. If you are using a tree zone, surround the tree zone with a ground zone of Loose Soil to account for the ground effects of that tree zone.

When should I enter in a ground zone?

Enter ground zones only where the ground differs from your ground-type default, except in the cases of roadway medians. TNM automatically sets the ground type to the default everywhere outside of your ground zones. In addition, only enter ground zones for relatively large patches of ground. For example, if several local streets intervene between source and receiver, enter them as ground zones if they cover a sizable fraction (20 to 30 percent or more) of the intervening ground, especially if they lie midway between source and receiver. Do not bother being too precise when locating the exact edges of ground zones. Note that TNM automatically places pavement along each roadway, depending upon the roadway's width. For this reason, ground zones are not needed to define pavement. Be careful not to allow two ground zones to touch one another.

What is the definition of field grass?

Field grass is described as high and thick meadow grass with dense roots. It is more absorptive than lawn.

How do I model hard and soft ground?

Use TNM's Hard Soil ground zone type, if you would like to model a generic acoustically hard ground surface. Use TNM's Lawn ground zone type, if you would like to model a generic acoustically soft ground surface.

Tree Zones

Does a tree zone automatically define the ground type?

No. If you want a ground type other than the default, you must use a ground zone to define the type of ground inside tree zones. If you are using a tree zone, surround the tree zone with a ground zone of Loose Soil to account for the ground effects of that tree zone.

How does TNM compute tree zone attenuation?

TNM computes tree attenuation per the International Standards Organization (ISO 9613-2, "Acoustics - Attenuation of Sound During Propagation Outdoors - Part 2). This standard requires that trees be sufficiently dense to completely block the view along the propagation path. This requires dense undergrowth as well as dense tree-top foliage. Do not include a TNM tree zone unless its vegetation is sufficiently dense.

When should I model tree zones?

TNM allows the user to model "tree zones." FHWA policy states that such zones should consist of long, wide regions of heavy, non-deciduous woods and undergrowth, not just individual trees or several rows of trees. The vegetation also must be sufficiently dense to completely block the view along the sound propagation path. This requires dense undergrowth as well as dense tree-top foliage. "Tree zones" should not be modeled unless they have this vegetative density.

Please refer to the Tree Zones section on Page 126 in the TNM Users Guide for more details.